CONSUMPTIVE USE OF IRRIGATION WATER
IN WYOMING

II. PROCEDURE

Consumptive Use Equation
Blaney and Criddle1 found that, disregarding the unmeasured factors,
consumptive use (U) varies with the temperature and daytime hours (F) and,
with the available moisture (precipitation, irrigation, and/or soil moisture
and ground water).

The computer will not print lower case letters, therefore, in Table IV (p. 22)
which is the computer printout of the consumptive use calculations, only capital
letters represent the lower case letters in the above discussion.

U. S. Weather Bureau records were used to define the growing season.
Perennial forage crops, such as alfalfa and grass, consume water for the
entire season as long as moisture is available for plant growth. A common
definition for growing season for forage crops is the number of days between
the last spring killing frost (28°F) and the first killing frost in the fall4.
Observations have indicated that forage crops begin to grow and consume
water as soon as the maximum daily temperatures stay well above the freezing
point for an extended period of days3. The season continues despite
later freezes. The end of the season occurs when the daily minimum temperature
repeatedly falls below freezing and mean dally temperatures
recede. The growing season for alfalfa and grass was assumed for this
report to be the period when mean daily temperatures are above 40°F.
Table II gives the forage crop growing season at the Wyoming weather
stations.

The growing season for other crops was estimated with the assistance
of specialists in the College of Agriculture, University of Wyoming. The
season for these crops is regarded as between the planting date and harvest.
Planting dates were determined to be ten days earlier than the 40°F date
(Table II) for small grains, fifteen days later than the 40°F date for
sugar beets, thirty days later for corn, forty days later for beans, and
fifty days later for potatoes. The harvest dates were estimated to be
August 1 for small grain, October 10 for sugar beets, October 1 for corn,
September 1 for beans, and September 15 for potatoes.

Consumptive Use Coefficients
The monthly consumptive use coefficients used for this report are
shown on Figures 1 through 7 (pp. These values were assumed to
be applicable to Wyoming after considering all available sources of data.

Effective Precipitation
Not all of the precipitation that falls on an area is effective for
meeting consumptive use needs of plants. This is because of many factors.
Showers of small intensity and duration are commonly evaporated. The
rainfall intercepted by plants before reaching the ground is likewise
evaporated directly from leaves. Runoff and groundwater accretion remove
portions of the rainfall from large storms, reducing the amount of water
retained in the soil for plant use from these storms.

A method suggested in the U. S. Bureau of Reclamation Manual and in
the U. S. Department of Agriculture Technical Bulletin No. 12751 to
estimate effective rainfall was used in this study. Mean monthly precipitation
is divided into one inch increments, and the effective rainfall is
calculated from the following percentages:

For an example, with an average monthly precipitation value of an inch and
one-half at a weather station, the effective rainfall for the month would
be 1.40 inches.

During some of the early months of the growing season, the effective
rainfall may exceed the consumptive use yielding a negative value for
irrigation requirement. In these cases, the consumptive irrigation requirement
was assumed equal to zero.

At the start of the growing season, regardless of the beginning date,
the amount of effective rainfall for the entire first month was subtracted
from the consumptive use to determine consumptive irrigation requirement.
This assumes that the precipitation prior to the start of the growing
season is stored in the soil and is available to satisfy the requirements,
At the end of the growing season, the fractional portion of the month within
the growing season was multiplied by the effective rainfall and this value
was subtracted from the estimated consumptive use.

Calculation of Consumptive Use
A computer program in Fortran IV for the Blaney-Criddle Method was
written for the computer at the University of Wyoming. The program is
included herein as Appendix 1.